2011-04-13

Opportunity Sol 789 (Spirit Sol 809)

Results from the previous drive were ... well ... suboptimal. And confusing, to boot. Opportunity detected excessive slip when climbing the first ripple, so we only made a few meters of progress. The confusing part is, we set her max allowed slip to 70%, and none of her reported slip numbers appeared to exceed that value. So we sort of know why she stopped, but not exactly.

While Matt looks into that, I survey the current state of the rover. The maximum reported slip numbers were still higher than they should have been, and we want to know whether it's safe to continue. Flying around for a while in RSVP, plus looking at the telemetry, clears things up nicely: we're just barely over the crest. In fact, when I look carefully at the rear HAZCAM and brighten the shadowed areas way up, I can just make out the crest of the ripple running right in front of our rear wheels. Another few cm and we'll be completely on the downhill side.

Foo. Well, the four wheels we can see aren't dug in, and the slip check turns out to have failed because of buggy -- well, let's say surprising -- behavior in the flight software that makes visodom slip checks generally more conservative than we thought they'd be. We told it to stop at 70% slip, but it was actually using a threshold of something like 52% when it stopped. So we're safe to continue, and that's what we do.

We don't go quite the same path as we'd picked out previously, though. It occurs to me today that our plan was to drive about 20m through small, but soft, transverse ripples, then hop a regular old ripple at a not particularly wide saddle point. In other words, after about 20m of picking up an unknown amount of slip, we're going to aim for a small target. And if we miss, we could have another Purgatory on our hands.

When I put it to myself in those terms, I can see what a bad idea it was all along. We could use visodom to compensate, but then we'd run out of time before we even got to the ripple, so that's pointless. Instead, I shoot for a few meters farther down the trough, where I can put a juicy patch of outcrop right in the IDD work volume. That's what Steve wants anyhow -- a little outcrop to IDD for the weekend -- and since it's the safest thing to do anyway, I'm happy to oblige.